op amp circuit

Archiver

Im sure this is an incredibly simple question, but is still holding up
progress on my project...........

my schematic shows +5V, -5V and ground, as values that are used in the
circuit............. +5V and ground of course, are easy enough.....my question
is,.....where do I get -5V for this cicuit?


Dave


[noparse][[/noparse]Non-text portions of this message have been removed]

Archiver

You need a negative power supply for -5V. There are several ways you can do
this.

First, op amps generally need more voltage than they will see on the
input/output and the regulation is not critical. So one answer is to use two
9V batteries connected in series. Use the "center" as ground. Now you have
+/- 9V for the op amp. If you really want +/-5 you could use a 7805 (5V) and
7905 (-5V) [noparse][[/noparse]or 78L05/79L05 if you don't need much current. That's the
easiest way.

If you have an AC mains supply, you can make one that again uses a center
tap on a transformer as ground and develop a +/- voltage from it.

If you are powered off a wall wart or something else you can't change, you
can use a simple DC to DC converter to make a negative voltage. This is
complicated and requires an oscillator, some diodes, and you'd probably want
a 7905 to keep it from going too high.

The other choice is to evaluate your op amp circuit and see if you can use
an op amp that has a "to the rail" output. Normal op amps can't go "to the
rail". So if you use 0V as V- and 5V as V+ the output range might be .5V to
4.3V (depends on the current output -- more current means less voltage).
However, some op amps are made to go to one or the other rail. For example,
an LM324 can go to ground, so you only need Ground and V+ (the output can go
to V+ - 1.5V). Of course, if you are doing something where you need negative
output, you can't avoid having a negative supply somewhere.

Read http://www.al-williams.com/wd5gnr/opampneg.htm Which has some links to
inverters and more discussion.

Regards

Al Williams
AWC
* Easy RS232 Prototyping: http://www.al-williams.com/awce/rs1.htm


>

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Original Message

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> From: samhell@s... [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=owqmWxQbVsxv1p-2NWva9i7ZmNvU2CiP3tyEcn4fIgsuNNB_SqLT15njFyD0SuFA3L3M5I0a10ln]samhell@s...[/url
> Sent: Sunday, August 26, 2001 3:08 AM
> To: basicstamps@yahoogroups.com
> Subject: [noparse][[/noparse]basicstamps] op amp circuit
>
>
> Im sure this is an incredibly simple question, but is still
> holding up progress on my project...........
>
> my schematic shows +5V, -5V and ground, as values that are used
> in the circuit............. +5V and ground of course, are easy
> enough.....my question is,.....where do I get -5V for this cicuit?
>
>
> Dave
>
>
> [noparse][[/noparse]Non-text portions of this message have been removed]
>
>
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